refactor!: design overhaul (#1)

* refactor!: use atom functions

* docs: reformat api reference

* docs: showcase atom functions

* docs: remove unused import

* refactor: finalize updates

* chore: misc edits

src/map.luau

@@ -0,0 +1,54 @@
+local atom = require(script.Parent.atom)
+local subscribe = require(script.Parent.subscribe)
+local types = require(script.Parent.types)
+type Atom<T> = types.Atom<T>
+type Source<T> = types.Source<T>
+
+type Map =
+	(<K0, V0, K1, V1>(atom: Source<{ [K0]: V0 }>, mapper: (V0, K0) -> (V1?, K1)) -> Source<{ [K1]: V1 }>)
+	& (<K0, V0, V1>(atom: Source<{ [K0]: V0 }>, mapper: (V0, K0) -> V1?) -> Source<{ [K0]: V1 }>)
+	& (<K0, V0, K1, V1>(atom: Source<{ [K0]: V0 }>, mapper: (V0, K0) -> (V1?, K1?)) -> Source<{ [K1]: V1 }>)
+
+local function map<K0, V0, K1, V1>(itemsAtom: Source<{ [K0]: V0 }>, mapper: (V0, K0) -> (V1?, K1?)): Source<{ [K1]: V1 }>
+	local mappedAtom = atom({})
+	local mappedAtomRef = setmetatable({ current = mappedAtom }, { __mode = "v" })
+	local prevMappedItems: { [K1]: V1 } = {}
+	local unsubscribe
+
+	local function listener(items: { [K0]: V0 })
+		if not mappedAtomRef.current then
+			return unsubscribe()
+		end
+
+		local mappedItems = table.clone(mappedAtomRef.current())
+		local mappedKeys = {}
+
+		for key, item in items do
+			local newItem, newKey = mapper(item, key)
+			if newKey == nil then
+				newKey = key :: any
+			end
+			if mappedItems[newKey :: K1] ~= newItem then
+				mappedItems[newKey :: K1] = newItem :: V1
+			else
+				mappedKeys[newKey] = key
+			end
+		end
+
+		for key in prevMappedItems do
+			if mappedKeys[key] == nil and mappedItems[key] == prevMappedItems[key] then
+				mappedItems[key] = nil
+			end
+		end
+
+		prevMappedItems = mappedItems
+		mappedAtom(mappedItems)
+	end
+
+	unsubscribe = subscribe(itemsAtom, listener)
+	listener(itemsAtom())
+
+	return mappedAtom
+end
+
+return map :: Map